This invention relates to a novel intelligent on-line pulp digester control system. More particularly, this invention pertains to a novel intelligent control system which employs heat and ion mobility spectrometry to automatically, rapidly and continuously determine on-line in real time the physical and chemical characteristics of feedstock being fed into a pulp digester and thereby maximize process efficiencies and minimize chemical and energy consumption.
Many pulp mills use a chemical pulping process for producing pulp from wood. In such a process, it is important that the physical and chemical characteristics of the wood chip feedstock that is fed into a pulp digester be taken into account in order to ensure consistent and optimal quality pulp production. The chemical pulping process includes a digester which is a vessel that utilizes steam and chemicals in the form of xe2x80x9cliquorxe2x80x9d to xe2x80x9ccookxe2x80x9d the wood chips under pressure and break them down to cellulose and hemi-cellulose fibres. Currently, there is no accurate or feasible way to measure the physical and chemical characteristics of the wood chip feedstock, including moisture and wood species type, continuously and on-line. The physical and chemical characteristics of the wood chip feedstock must be estimated according to empirical formulae.
The moisture content of the wood chips feeding a digester depend on such factors as their source (residual chips or whole log chipping), the wood species involved, the retention time of the wood chips on the wood chip piles and the time of year. These variables often result in wood chip moisture deviations of between 5 and 10 percent, which is significant for pulp production purposes. The practice is to err on the conservative side in estimating moisture levels.
The nature of wood species in the wood chips also is important because it too affects the xe2x80x9ccookingxe2x80x9d process. Certain species of wood chips require higher levels of alkali chemicals to digest the chips and break them down into cellulose and hemi-cellulose fibres. Because of these significant variables and the inability with current instrumentation to continuously monitor chip moisture content and wood species type on-line, operators of pulp mills tend to run higher than required liquor to wood ratios in the digester in order to ensure consistent quality and that the produced pulp will meet required specifications. The downside of running elevated liquor to wood ratios and active alkali charges, however, is that more than necessary levels of chemicals and high levels of steam are required to heat the liquor, which in turn increases energy costs. Running less than conservative liquor to wood chip ratios and active alkali charges runs the risk of reducing wood pulp quality due to loss of circulation and the occurrence of dirty or uncooked blows. On balance, to ensure high quality pulp, it is preferred to follow a conservative approach and run at higher ratios, which are more costly.
Apart from the handicap of having to operate the digester empirically under conservative parameters which raise energy and processing chemical costs, there is an inherent shortcoming with current testing procedures. Currently, there is no instrumentation or methodology available which can rapidly monitor wood chip feedstocks on-line and determine wood species and moisture content. The common procedure to determine wood species is to run tests on the pulp that is produced by the digester to determine if the pulp meets specifications for a specific grade type. This is known as a xe2x80x9cfeedbackxe2x80x9d testing approach. The problem with this approach is that in many cases approximately three hours of pulp production are jeopardized if the resultant pulp quality is not up to specifications. If the pulp does not meet specifications, it must be downgraded to a lower grade which attracts a lower price.
It would be highly advantageous if lower amounts of chemicals and less liquor were used in a digester cook, so that less steam would be required to heat the liquor to cooking temperature. Typically, if this could be done, it is conservatively estimated that liquor to wood ratios would be reduced by a minimum of 6 percent on hemlock or Douglas fir wood chips, and by 11 percent on western red cedar wood chips. This would result in a reduction in energy consumption per cook of approximately 4 to 5 percent on hemlock or Douglas fir wood chips and of about 8 percent on western red cedar. Benefits will be mill specific and dependent upon the wood species they cook, the moisture content of their chips and their current cooking strategy.
For the foregoing reasons, it would be extremely advantageous if a continuous rapid xe2x80x9cfeed-forwardxe2x80x9d on-line testing procedure were available so that wood chip species and moisture content could be continuously monitored on-line. This information would facilitate improved chip species control, active alkali charge and liquor to wood ratio control and would ensure consistent production of pulp to specified standards. In addition, to the economic benefits of improved control, the environmental impact of the process would be reduced.
In recent years, pulp mill owners have become increasingly aware of the importance of accurately determining the wood species of the feedstock. Previously, it was common practice, particularly in northwestern North America, to mix different wood species together. However, different wood species have different wood fibre properties. For example, Douglas fir has coarse, stiff fibres which, when pulped, forms an open, free-draining pulp mat with very high tear strength. Western red cedar, on the other hand, has flexible flat fibres which, when pulped, yields a closely grouped fibre mass which drains very slowly but has excellent tensile strength properties. Certain pulps are made from a mixture of western hemlock and balsam feedstock. The properties of these blends fall between those of Douglas fir and western red cedar. In recent years, purchasers have become increasingly demanding in asking for specific quality pulps made from specific wood species. It is therefore important for pulp producers to be able to control the wood fibre type. This is done by segregating wood species and controlling the species of wood in the feedstock. The benefits of segregating wood chips according to their species applies equally to both hardwoods and softwoods.
For the foregoing reasons, there is a strong and long felt need for a quick and accurate intelligent feed forward control system for determining moisture content and wood species of wood chips on-line as the wood chips are fed via a conveyor from a wood chip source to a digester to ensure consistent, optimum quality pulp production with minimum energy and chemical consumption. Real time measurement of the physical and chemical characteristics of the wood chip feedstock (moisture content and species) would enable the minimization of chemical and energy requirements.
Ion mobility spectrometry (IMS) is relatively recently developed technology that is capable of separating ionized compounds based on differences in their drift velocity through a gas under an applied electric field. This IMS technique has the ability to produce, in a matter of milliseconds, a characteristic spectrum of a specific series of high molecular weight compounds. It is known, for instance, that IMS can produce identifiable signatures for such items as narcotics and explosives and hence it is being developed for use by customs, airlines and police forces to detect such substances.
Initial tests have been carried out to determine if IMS can be used to identify different bulk wood species, such as lumber, on a static basis. A report on these tests, entitled xe2x80x9cRapid Characterization of Wood Species by Ion Mobility Spectrometryxe2x80x9d, was published by A. H. Lawrence on Feb. 2, 1989 at the 75th Annual Meeting of the Technical Section of the Canadian Pulp and Paper Association. Some tests were carried out in the xe2x80x9cpositive modexe2x80x9d and some in the xe2x80x9cnegative modexe2x80x9d. Positive mode is when polarity of the electric field is positive, i.e. positive ions are present in the detection mode. Negative mode is when the polarity of the electric field is negative, i.e. negative ions are present in the detection mode. The initial static tests demonstrated that it was possible to distinguish some wood species from other wood species provided the tests were conducted in both modes. However, a number of variable parameters occurred that initially appeared to make the IMS technology unacceptable for use in the lumber industry. First of all, sampling and analyzing the wood species by an IMS device took several seconds which made it unsuitable for fast moving conveyors used in saw mills. Secondly, it seemed that only certain types of wood species could be identified. Thirdly, it was not clear how the IMS device would work in a typical saw mill environment where obscuring contaminants such as sawdust, other types of particles, and vapors from both machinery and wood were present.
U.S. Pat. No. 5,071,771, Barbour et al., granted Dec. 10, 1991, discloses a method and apparatus for assessing a wood sample and producing an ion mobility signature representing the wood sample. Respective signatures can be compared to identify the specific wood species. The method involves producing an ion drift time signature for a specific wood species. The method comprises heating at least a portion of the wood sample to a temperature in the range of about 220xc2x0 to 350xc2x0 C. This desorbs and produces trace vapors from the wood sample. The vapors are ionized at a temperature in the range of about 220xc2x0 to 350xc2x0 C. The ions are pulsed through a gate into a drift region. The time of arrival of the ions and the ion flux for each pulse are measured with a collector electrode which is located at the end of the drift region. This produces an ionic signal which is amplified. The signal is averaged to provide an ion drift time signature for the specific wood sample.
The present invention is to be able to analyze representative wood chip samples for moisture content and species type on-line within a very short space of time on a conveyor belt delivering the wood chips to a digester. The subject invention provides a method of optimizing chemical and energy usage required in a pulping process by producing on-line in the pulping process an ion drift time signature representing a wood chip species and moisture content, comprising the steps of rapidly heating representative wood chips with a laser, such as a carbon dioxide laser, to a temperature in the range of about 220xc2x0 to 350xc2x0 C., and utilizing an ion mobility spectrometer.
The invention is directed to a process of utilizing accurate, real time measurements of the moisture content and species of wood chips on-line in a pulping control process comprising: (a) conveying wood chips from a wood chip storage facility to a digester; (b) scanning the wood chips as they are conveyed to determine their species and moisture content by heating the wood chips and sensing the moisture content and characterizing the species of the wood chips by species signatures obtained by an ion mobility spectrometer; and (c) regulating steam flow and chemicals supplied to the digester according to the moisture content and the characterization of the wood chip species.
The rate of conveyance of the wood chips from the storage facility to the digester can be controlled by a wood chip rate control mechanism. The wood chip storage facilities discharge devices can be regulated according to the characterization of the wood chip species. The wood chips can be heated with a laser beam. Representative samples of wood chips can be periodically removed from the wood chips being conveyed to the digester for analysis by the ion mobility spectrometer. Removal can be done with a robotic arm at a transfer point. The data from the ion mobility spectrometer can be transmitted to a programmed microprocessor in a distributed control system which can continuously control rate of steam and chemical supply to the digester to minimize steam and chemical use and optimize digestion time of the wood chips.
The invention in another embodiment is directed to a process of utilizing accurate and real time measurements of the moisture content and species of wood chips in an on-line pulping control process comprising: (a) conveying wood chips from a wood chip storage facility to a digester; (b) controlling the rate of flow of the wood chips with a wood chip rate control mechanism; (c) sampling the wood chips at an optimum transfer point using a robotic arm or continuously monitoring wood chips on the conveyor belt; (d) scanning the wood chips provided by the robotic sampler to determine their species and moisture content by heating the wood chips and sensing the moisture content and characterizing the species of the wood chips by species signatures obtained by an ion mobility spectrometer; and (e) regulating steam flow and chemicals supplied to the digester according to the characterization of the wood chip species.
The distributed control system of the invention can continuously monitor and adjust process targets to optimize wood species control. Optimal chip sampling sites can be determined for utilizing the moisture content and species data obtained by the ion mobility spectrometer in real time mode. The data obtained from the ion mobility spectrometer can be transmitted to a control system which can adjust wood chip reclaim speed to regulate chip species control. Wet wood mass flow rate can be determined by an on-line chip weightometer or from the digester fill weights or chip meter RPM.
The invention in a further embodiment is also directed to a process of characterizing moisture and species of wood chips in real time in a pulp mill wood chip feedstock and continuously controlling feedstock rate, and chemical and steam consumption by removing a representative wood chip sample from the feedstock, heating the wood chip sample with a laser to produce trace vapours from the wood chip sample, ionizing the trace vapours, measuring the time of arrival of the ionized vapours at a collector electrode, producing an electrical data signal signature representative of the moisture and wood chip species in the feedstock, transmitting the electrical data signal to a distributed control system which regulates a wood chip silo variable frequency drive according to a predetermined set point, regulates liquor to wood ratio according to a second predetermined set point, and regulated active alkali to dry wood ratio according to a third predetermined set point.
It is understood that the principles of this invention can be applied to pulping processes that utilize non-wood sources such as bagasse, reeds, straw and other fibre sources.